What is the molarity of a solution that contains 0.0345 mol nh4cl in exactly 400 ml of solution?

The molarity of a solution is the number of moles of a substance per liter of the solution.

400 mL becomes 0.400 liters

You have 0.0345 moles of ammonium chloride NH4Cl, so take the number of moles over the number of liters to find the answer.

0.0345 moles/0.400 liters = 0.0863 moles/liter

0.0863M = molarity of the solution